Packet-based communications systems have been developed for transmitting digital signals. In some communications systems, the communications channel is noisy and can introduce errors into the packets, decreasing the reliability of the communications. In others, such as the internet, the communications channel can lose packets altogether. Systems have been developed to increase the reliability of such communications systems.
These systems include some kind of error detection and correction mechanism. For example, in U.S. Pat. No. 5,677,918, entitled “METHOD AND DEVICE FOR EFFICIENT ERROR CORRECTION IN A PACKET-SWITCHED COMMUNICATION SYSTEM” and issued Oct. 14, 1997 to Tran et al., a packet switched communication system is disclosed in which packets of data are divided into segments. A first error detection code is generated over each segment, and a second over the packet as a whole. The segments making up the packet are then transmitted over the communications channel. A receiver receives the segments and checks the error detection codes in each segment and the packet. If an error is detected in a segment or the packet as a whole, the receiver sends a negative acknowledgement to the transmitter, which, in response, retransmits the erroneous segment, or the entire packet, until it has been successfully received.
This system depends on negative acknowledgements and retransmission to correct erroneous packets. This requires additional communications, reducing the overall throughput, and requires a back channel from the receiver to the transmitter to communicate the negative acknowledgements. Such a back channel is not always available. For example, broadcast systems such as terrestrial, cable and satellite television transmission system are one-way systems only.
In European patent publication EP 1 014 730 A1, entitled “FORWARD ERROR CORRECTION AT MPEG-2 TRANSPORT STREAM LAYER” and published Dec. 22, 1999 for Wan et al., an MPEG packet system is disclosed in which a plurality of packets, termed a supergroup, is accumulated, and an error correction code is generated over the data in the supergroup. The error correction code data is then packetized and appended to the packet stream as optional packets. Appropriately equipped receivers may receive the MPEG packets and the optional appended error correction code packets and perform error correction on the received supergroup of MPEG packets.
U.S. Pat. No. 5,122,875, entitled “AN HDTV COMPRESSION SYSTEM” issued Jun. 16, 1992 to Raychaudhuri et al., discloses a packet communications system in which high priority and low priority MPEG data are transmitted in respective packet streams on separate modulated carriers. Each packet in both packet streams includes an error detection code. In addition respective forward error correction codes are calculated over successive blocks of packets in both packet streams then appended to the packet streams. At the receiver, the forward error correction code is used to correct for errors in the respective received packet streams. Then the error detection code in each packet in both packet streams is analyzed to determine if an error remains in that packet. If such as error is detected, then some form of error concealment is performed. For example, for video MPEG data, should an erroneous packet be detected, the image area represented by that packet is not updated to prevent the erroneous data from distorting the image.
It is known that every error correction code can correct a maximum number of errors. The number of errors correctable by the code is related to the number of code bits relative to the number of data bits protected, i.e. to correct more errors requires more code bits, which, in turn, increases the overhead and lowers the throughput of the communications system. While it is possible to select an error correction code which is strong enough to correct for the loss of one or more packets (e.g. in EP 1 014 730 A1 up to nine lost packets and up to 4½ ‘errored’ packets may be corrected), there is always a limit to the number of lost packets which may be corrected. If any more than that number is lost, then all of the packets in the block of packets over which the error correction code was originally calculated are compromised. In such a case, in order to minimize potential image degradation, none of the packets in the group are used in generating an image. However, it is desirable that, even if more than the predetermined number of packets is lost, the data in the remaining packets be made available for use.